Programmable SONET framing

ABSTRACT

A synchronous optical network (SONET) framer includes a frame dimension unit and a programming interface. The frame dimension unit can be programmed with a frame dimension through the programming interface. The SONET framer converts a data stream to and/or from a frame format based on the frame dimension programmed into the frame dimension unit. For instance, in various embodiments, a SONET framer can be programmed to support a variety of SONET frame sizes and to provide a number of testing and design advantages.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of prior application Ser. No.09/918,896, entitled, PROGRAMMABLE SONET FRAMING, filed Jul. 30, 2001now U.S. Pat. No. 7,075,953, priority from the filing date of which ishereby claimed under 35 USC §120.

FIELD OF THE INVENTION

The present invention pertains to the field of networking. Moreparticularly, this invention relates to programmably framing SynchronousOptical Network (SONET) frames.

BACKGROUND

Fiber optics have provided a substantial increase in the volume of datathat networks can carry. Synchronous Optical Network (SONET) is astandard that defines data transmissions over fiber optics. SONETdefines a number of different data rates for different levels ofservice. A typical SONET network can carry from about 52 megabits persecond up to about 10 gigabits per second over a single optical fiber.In comparison, a typical analog modem operating at maximum efficiencycan achieve a mere 56 Kilobits per second. At the 10 gigabit SONET rate,a single optical fiber can carry enough data to handle well over 100,000simultaneous voice calls. SONET networks are likely to carry even largervolumes of data in the future.

Data in a SONET network is often processed in electrical form. Forinstance, a SONET network may be made up of a web of optical fibersinterspersed with routers and/or switches. The routers and/or switchesdetermine which path data should take to reach a particular destination.Optical data is usually converted to electrical form before beingprocessed by a router or switch. If a router or switch determines thatdata is to continue on through the SONET network, the data is convertedback to optical form before being passed on to the next router orswitch. Data may pass through many routers and/or switches between asource and a destination.

Data is also converted to and/or from optical form at the edges of aSONET network. For example, routers and/or switches may connect one ormore electrical networks to one or more SONET networks. Data may startout in electrical form to travel over wires making up an electricalnetwork, get converted by routers and/or switches back and forth betweenelectrical and optical form while traveling among various SONET networksand/or additional electrical networks, and eventually travel inelectrical form over wires leading to a destination.

Each time data enters or leaves a SONET network, or gets processed inelectrical form within a SONET network, the data is usually convertedto/from a particular SONET frame format. The frame format depends on thephysical characteristics of the particular SONET medium. Different mediacan support different sizes of SONET frames, and therefore differentvolumes of data.

In the past, the electrical hardware used to convert data to and/or fromSONET frames has been matched to the corresponding physical SONETmedium. That is, if the SONET medium supports 52 megabits per second, aframer is used that converts frames at 52 megabits per second. If theSONET medium supports 10 gigabits per second, a framer is used thatconverts frames at 10 gigabits per second. If, for any reason, adifferent data rate is needed or desired, new hardware is required,often requiring considerable additional expense.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the present invention are illustrated in the accompanyingdrawings. The accompanying drawings, however, do not limit the scope ofthe present invention. Similar references in the drawings indicatesimilar elements.

FIG. 1 illustrates one embodiment of a single-plane SONET frame.

FIG. 2 illustrates one embodiment of a composite SONET frame having 192planes.

FIG. 3 illustrates one embodiment of a router.

FIG. 4 illustrates one embodiment of a SONET networking card.

FIG. 5 illustrates one embodiment of a SONET PMD card.

FIG. 6 illustrates one embodiment of a SONET output processor.

FIG. 7 illustrates one embodiment of simulation environment.

FIG. 8 illustrates one embodiment of hardware testing.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the presentinvention. However, those skilled in the art will understand that thepresent invention may be practiced without these specific details, thatthe present invention is not limited to the depicted embodiments, andthat the present invention may be practiced in a variety of alternateembodiments. In other instances, well known methods, procedures,components, and circuits have not been described in detail.

Parts of the description will be presented using terminology commonlyemployed by those skilled in the art to convey the substance of theirwork to others skilled in the art. Also, parts of the description willbe presented in terms of operations performed through the execution ofprogramming instructions. As well understood by those skilled in theart, these operations often take the form of electrical, magnetic, oroptical signals capable of being stored, transferred, combined, andotherwise manipulated through, for instance, electrical components.

Various operations will be described as multiple discrete stepsperformed in turn in a manner that is helpful in understanding thepresent invention. However, the order of description should not beconstrued as to imply that these operations are necessarily performed inthe order they are presented, or even order dependent. Lastly, repeatedusage of the phrase “in one embodiment” does not necessarily refer tothe same embodiment, although it may.

The present invention provides the flexibility to program the size of aSynchronous Optical Network (SONET) frame. In one embodiment, hardwareused to convert a data stream to and/or from SONET frames uses countersto count the number of byte positions in a SONET frame. The presentinvention provides programming access to one or more of the counters sothat the number of bytes per frame can be changed.

By programming the size of SONET frames, a variety of standard SONETdata rates may be supported by a single SONET framer. For instance, if adata port on a router is originally coupled to a SONET medium thatsupports one data rate, and the SONET medium is later changed to adifferent data rate, it may be possible to reprogram the SONET framer tosupport the new data rate, saving the cost of replacing the SONET framerwith new hardware. A user may even be able to reprogram the framerhimself or herself, avoiding the cost and inconvenience of a servicecall. Furthermore, a supplier may be able to support multiple data rateswith one piece of hardware, eliminating the cost and inconvenience ofstocking different hardware for each data rate.

Programmable frame sizes can also be used to improve a variety oftesting and hardware development situations. For instance, high speedSONET hardware tends to be fairly complex. Designs are usually createdand tested in a simulation environment prior to costly fabrication. Atthe 10 gigabit data rate, a single SONET frame includes about 155thousand bytes. A software simulator may take hours to simulate theframing of a single frame of data. Once the design of the framer hasbeen tested and verified, continuing to simulate the framing of hugeframes of data may be unnecessary for the verification of othercomponents of the design. By programming the framer for a smaller framesize, the simulation time needed to design and test the other componentscan be substantially reduced.

Programmable frame size can similarly improve the performance of ahardware emulator. A hardware emulator is made up of hardware elementsthat can be programmed to represent the functionality of circuitdesigns. For instance, a circuit design can be written in a hardwaredescription language, such as Verilog, and loaded into an emulator tomodel the design. Hardware emulators tend to be much faster thansoftware simulators. But, as fast as hardware emulators are, theprogrammable hardware elements tend to be less efficient than actualSONET hardware. At a high SONET data rate, such as the 10 gigabit rate,even an emulator may be bogged down by the volume of data, and maybenefit from smaller frame sizes.

Programmable frame size can be helpful when testing the actual hardwareof a SONET framer as well. For instance, a logic analyzer can be used tocapture and analyze data from the actual hardware. At the 10 gigabitrate, the volume of data in a single frame is so large that a logicanalyzer may only be able to capture part of a single frame, making itvery difficult to test the functionality of the hardware across frameboundaries. By programming the framer for smaller frame sizes, the logicanalyzer may be able to capture and analyze several frames of data,exposing the functionality at frame boundaries and allowing the framerto cycle through more operational states in a shorter period of time.

Programmable frame size can also be helpful when developing hardwareand/or software surrounding a SONET framer. For instance, if a newhardware component for processing SONET overhead is needed, and theoverhead processor needs to operate on the same data stream as the SONETframer, it may be desirable to simulate or emulate the overheadprocessor and couple the modeled design to the actual SONET framer. Anynumber of simulator or emulator interfaces can be used. The simulator oremulator may not be fast enough to handle the maximum data ratesupported by the framer. In which case, the framer could be programmedto a lower data rate to match the speed of the simulator or emulator.

FIG. 1 illustrates one embodiment of a single-plane SONET frame. TheSONET frame is 9 rows high by 90 columns wide. Each entry in the frameis an 8 bit byte. Transmitting the data in a frame begins at row 1,column 1 and ends at row 9, column 90. Bits of data are transmittedserially, one at a time, as pulses of light in an optical fiber. Theframe is read across rows from left to right, one row at a time.

Frames are written in the illustrated format so that overhead bytes andpayload bytes are easily identified. Bytes in the first three columns,columns 110, are reserved for overhead bytes, and bytes in the remainingcolumns, columns 120, are payload bytes. When the data is transmittedserially, the first three bytes are overhead, the next 87 bytes arepayload, the next three bytes are overhead, the next 87 bytes arepayload, and so on.

Some additional byte positions in one column among the payload bytes mayalso be used for overhead. In the illustrated embodiment, the additionaloverhead bytes 130 are in column 4 of the payload section. In alternateembodiments, the additional overhead bytes can be in any column in thepayload section, and may not occupy an entire column: All of theoverhead bytes, including those in columns 110, are used to managevarious operations in a SONET network, such as routing, error checking,and the like.

FIG. 2 illustrates one embodiment of a composite SONET frame. Thecomposite frame is comprised of multiple planes identical to thesingle-plane frame illustrated in FIG. 1. That is, each plane in thecomposite frame includes 9 rows and 90 columns, with up to 36 bytes ofoverhead. In the illustrated embodiment, there are 192 planes in theframe.

The composite frame is read from front to back, and then left to right.That is, the byte in row 1, column 1 is read from each plane, startingwith plane 1 and ending with plane 192. Then, the bytes in row 1, column2 are read from all 192 planes, followed by the bytes in row 1, column 3from all 192 planes, and so on. As the composite frame is read serially,the data stream comprises 9 iterations (one iteration for each row) of576 bytes of overhead (from 192 planes times 3 overhead bytes per row)followed by 16704 bytes of payload (from 192 planes times 87 payloadbytes per row). Somewhere among each 16704 bytes of payload there arealso up to 192 continuous bytes of payload that are used for overhead,depending on the logical construction of the composite frame.

The composite frame of FIG. 2, like the single-plane frame of FIG. 1, isa single unit in the SONET network. That is, no matter how many planes aframe has, one frame is transmitted every 125 micro seconds, or 8000times per second. By increasing the size of a frame by adding moreplanes, the data rate increases. For instance, the data rate for thesingle-plane frame of FIG. 1 is 810 bytes per frame times 8 bits perbyte times 8000 frames per second, which equals 51.84 Megabits persecond. For the 192-plane frame of FIG. 2, the data rate is 810 bytesper plane times 192 planes per frame times 8 bits per byte times 8000frames per second, which equals 9.95328 Gigabits per second.

The frame format for the 192 plane frame of FIG. 2 is referred to asoptical carrier level 192 (OC-192). The optical carrier levelcorresponds to the number of planes in the frame. OC-192 is a standardSONET frame size that provides the 10 gigabit data rate. Other standardSONET frame sizes that provide lower data rates include OC-3 (156megabits/second), OC-12 (625 megabits/second), and OC-48 (2.5gigabits/second).

A SONET framer converts a data stream to a stream of SONET frames byfilling available payload bytes locations with data bytes from the datastream and filling byte locations reserved for overhead with overheadbytes generated by other components. A SONET framer recovers a datastream in the opposite way.

In one embodiment, a SONET framer includes three counters, a rowcounter, a column counter, and a plane counter. The values of thecounters define the current byte position in a frame, and depending onthe byte position, the framer knows whether a byte is payload oroverhead.

FIG. 3 illustrates one embodiment of a router 300 that can takeadvantage of the present invention. Router 300 includes a number ofnetworking cards 320. In the illustrated embodiment, each network cardincludes three physical medium dependency (PMD) cards 330. Each PMD card330 has input/output ports to couple the router 300 to a physicalnetwork medium. PMD cards 330 may couple the router to any number ofelectrical wire networking media, wireless networking media, and, ofcourse, optical fiber media for SONET networking. The optical fibermedia may include fibers that support a variety of optical carrierlevels, such as OC-3, OC-12, OC-48, and OC-192.

Router 300 also includes data fabric 310. As data is received from aphysical networking media, the corresponding PMD card 330 and networkingcard 320 convert the data to a format that is usable by data fabric 310.Data fabric 310 then directs the data to one of the various outputports. A networking card 320 and PMD card 330 corresponding to theselected output port convert the data to a format supported by thephysical networking medium.

When physical network media are added, deleted, or replaced, thenetworking cards 320 and PMD cards 330 are changed to accommodate thephysical media. For changes in SONET optical carrier level, the presentinvention makes it possible to change the card by simply reprogrammingthe networking card 320 to support the new physical media rather thanreplacing the entire card.

Except for the teachings of the present invention, router 300 is tendedto represent a wide variety of networking devices known in the art.Alternate embodiments may include any number of input/output portsdispersed among any configuration of networking cards and PMD cards. Inalternate embodiments, the present invention may similarly be used withany number of SONET devices, such as signal regenerators, switches, andthe like.

FIG. 4 illustrates one embodiment of a SONET networking card 410 in moredetail. The networking card 420 includes a router interface 420, andthree pairs of SONET processors 440 and corresponding PMD slots 430 tocouple the router to three SONET networks. In the past, if any one ofthe SONET networks changed, it would be necessary to replace the entirecard, potentially at great expense. However, with the present invention,a single card can be programmed to support several different SONETphysical media. Rather than replacing the entire card when one physicalmedium changes, the corresponding SONET processor 440 can bereprogrammed to support the new medium.

Alternate embodiments of SONET networking cards may include any numberof PMD card slots. Generally, the more slots a networking card provides,the more expensive the card is to replace, making reprogrammability thatmuch more desirable.

FIG. 5 illustrates one embodiment of a SONET PMD card 510 in moredetail. PMD card 510 is intended to represent a wide variety of suchdevices known in the art. Such devices tend to be relatively simple andinexpensive. In the illustrated embodiment, the card includes anelectro/optical converter 530 and a clock generator 540. Electro/opticalconverter 530 converts framed data 550 to and from optical data 520.Converter 530 is “matched” to the physical medium of the network. Thatis, SONET data is transmitted synchronously, at a constant data rate.Converter 530 sends and receives data at both the electrical and opticalsides according to the data rate dictated by the optical carrier levelof the physical medium.

Clock generator 540 generates clock signals 560 for internal use and forthe corresponding SONET processor. In one embodiment, clock signals 560include a frame synchronization signal and a data clock. SONET framesare sent at a constant synchronous rate, 8 kHz, no matter what theoptical carrier rate is. In which case, a universal framesynchronization signal is provided throughout a SONET network. Clockgenerator 540 passes the frame synchronization signal to the SONETprocessor and derives the data clock from the frame synchronizationsignal. The data clock is dictated by the data rate of the opticalcarrier level of the physical network medium. Since SONET frames arebuilt one byte at a time, the data clock is usually derived such that itcycles once for each byte of data. In which case, the data clock forOC-192 would be approximately 1.2 gigahertz.

FIG. 6 illustrates one embodiment of a SONET processor, processor 600,for processing an output data stream. The same processing happens inreverse for an input data stream. In the illustrated embodiment,processor 600 includes overhead processor 610 and framer 620. Overheadprocessor 610 performs a wide variety of SONET processing known in theart. Framer 620 combines data stream 628 from the router with variousoverhead bytes from overhead processor 610 to provide framed data 660 tothe PMD card at the constant data rate dictated by the optical carrierlevel of the SONET medium.

In order to provide framed data 660 at the dictated rate, framer 620receives a frame sync signal 624 and a data clock 626. Frame sync 624may be received from the PMD card, from some other source, or, in analternate embodiment, may be generated internally. Similarly, data clock626 may be received from the PMD card, from some other source, or, in analternate embodiment, may be generated internally.

Framer 620 starts a new frame for each frame sync signal 624. That is,row counter 630, column counter 640, and plane counter 650 all startfrom one. Plane counter 650 is incremented for each clock cycle of dataclock 626. Column counter 640 is incremented each time plane counter 650reaches its maximum value and returns to one. Row counter 630 isincremented each time column counter 640 reaches its maximum value andreturns to one. Depending on the byte position in the SONET frameindicated by the values of the three counters, framer 620 loads either abyte from data stream 628 or a byte of overhead into framed data 660 foreach clock cycle of data clock 626.

Except for the teachings of the present invention, framer 620 representsany of a number of such devices known in the art. In alternateembodiments, any number of techniques can be used to identify bytelocations in frames. For instance, one alternate approach uses twocounters rather than three, and represents byte locations in SONETframes in two dimensions.

According to the teachings of the present invention, one or more of thecounters within framer 620 are programmable. Inventive framer 620 alsoincludes program interface 622 to program the programmable counter(s).Program interface 620 may be, for instance, a data bus coupled to aprocessor. In which case, software can be used to program the counter(s)by writing to particular address spaces corresponding to the counter(s).Alternately, program interface 620 may represent any number ofinterfaces, including one or more dip switches or the like.

Where plane counter 650 is programmable, framer 620 can be programmed tosupport multiple standard SONET data rates. For instance, in oneembodiment, the default setting for plane counter 650 is 192, to supportOC-192. However, by reprogramming plane counter 650 to 48, 12, or 3, theSONET processor 600 can support OC-48, OC-12, or OC-3, respectively. Rowcounter 630 and column counter 640 do not need to be programmable tosupport any of these standard data rates because the plane dimensionsare 9 rows by 90 columns for each standard data rate.

If the framer is only programmable to support four different standardSONET data rates, program interface 622 could comprise a pair of dipswitches to represent the four possible modes of operation. Alternately,software could write a two bit register value to the framer to selectone of the four possible modes of operation. Of course, any number ofapproaches can be used for program interface 622, including directlywriting the desired maximum number of planes to the programmable planecounter.

Under certain circumstances, it can be useful to program framer 620 togenerate non-standard frames. In which case, plane counter 650 may beprogrammed to a non-standard number of planes. Furthermore, row counter630 and/or column counter 640 may also be programmable to providenon-standard plane dimensions. For instance, each time the number ofrows is reduced by one, the frame size is reduced, and therefore thevolume of data per frame is reduced, by almost 17 thousand bytes.Reducing the volume of data may be helpful when testing or designingvarious components. However, reducing the number of rows may cut offsome needed overhead bytes. In which case, it may be more desirable tokeep the number of rows, but reduce the number of columns. By reducingthe maximum number of columns, the payload portion of a frame can betrimmed without loosing overhead bytes. In other situations, it may bebeneficial to change any combination of rows, columns, and planes.

FIGS. 7 and 8 illustrate some situations in which it can be useful toreprogram the frame size to various sizes, including non-standard framesizes. In FIG. 7, simulation environment 710 is used to model a circuitdesign 720. Simulation environment may include software simulation,hardware emulation, and the like. If circuit design 720 models theframing of a large SONET frame, it may take an excessively long-time tosimulate. For instance, in software simulation, an OC-192 frame mayseveral hours to simulate. Once framing has been tested and verified, itmay be possible to speed up the testing and verification of othercomponents in design 720 by programming the frame size to reduce thevolume of data being modeled.

Furthermore, if design 720 represents components that interact with aSONET framer, it may be beneficial to model the components inconjunction with the actual SONET framer hardware component 730 throughsimulation environment interface 740. Any number of co-simulationenvironments can be used to couple simulation environment 710 tohardware component 730. Again, it may be possible to speed up thetesting and verification of design 720 by programming the frame size toreduce the volume of data being modeled.

In FIG. 8, hardware component 810 includes a SONET framer. The hardwarecomponent is analyzed by logic analyzer 820. That is, logic analyzer 820provides various test vectors to hardware component 810 to cycle thehardware through various operational states. Logic analyzer captures andanalyzes the hardware responses to the test vectors. At the OC-192 datarate, the logic analyzer may only be able to capture and analyze aportion of a single frame. By reducing the frame size, logic analyzer820 may be able to capture data from multiple frames, thereby cyclingthrough more operational states more efficiently and in less time.

In one embodiment, the present invention, as described above, isimplemented as part of an application specific integrated circuit (ASIC)for SONET protocol processing. The software described above forprogramming frame sizes could be implemented in code running on anembedded controller, an external processor, or the like, as will becomprehended by a person skilled in the art. In another example, fieldprogrammable gate arrays (FPGAs) or static programmable gate arrays(SPGA) could be used to implement one or more functions of the presentinvention. In yet another example, a combination of hardware andsoftware could be used to implement one or more functions of the presentinvention.

Thus, programmable SONET framing is described. Whereas many alterationsand modifications of the present invention will be comprehended by aperson skilled in the art after having read the foregoing description,it is to be understood that the particular embodiments shown anddescribed by way of illustration are in no way intended to be consideredlimiting. Therefore, references to details of particular embodiments arenot intended to limit the scope of the claims.

1. An apparatus, comprising: a frame dimension unit; and a SONET framercoupled to the frame dimension unit and included on a networking cardhaving a plurality of SONET processors, the SONET framer configured toconvert a data stream to and/or from a frame format, wherein the frameformat includes a variable frame size provided by a programmable planecounter, row counter, and column counter included in the frame dimensionunit, the SONET framer further configured to support a first data rateby converting a data stream to and/or from the frame format based atleast in part on a first frame dimension programmed into theprogrammable plane counter, row counter, and column counter wherein theSONET framer is reprogrammable to support a second data rate based atleast in part on a second frame dimension programmed into theprogrammable plane counter, row counter, and column counter.
 2. Theapparatus of claim 1 wherein the frame dimension comprises aprogrammable number of planes per frame.
 3. The apparatus of claim 2wherein the SONET framer supports a plurality of standard SONET datarates corresponding to particular values of the programmable number ofplanes.
 4. The apparatus of claim 2 wherein the SONET framer supports arange of data rates corresponding to particular values of theprogrammable number of planes.
 5. The apparatus of claim 1 wherein theapparatus comprises a simulation environment, and wherein the framedimension is programmed to achieve a data rate supported by thesimulation environment.
 6. The apparatus of claim 5 wherein thesimulation environment comprises at least one of a software simulatorand a hardware emulator.
 7. The apparatus of claim 1 further comprising:a simulation environment interface to couple the apparatus to asimulation environment, said frame dimension to be programmed to achievea data rate supported by the simulation environment.
 8. The apparatus ofclaim 1 further comprising: a logic analyzer interface to couple theapparatus to a logic analyzer, said frame dimension to be programmed toachieve a volume of data per frame supported by the logic analyzer. 9.The apparatus of claim 1, further comprising an input/output (I/O) portcoupled to the SONET framer to provide the data stream.
 10. Theapparatus of claim 1, wherein the networking card further includes oneor more slots configured to be coupled to one or more physical mediumdependency cards.
 11. The apparatus of claim 1, wherein the networkingcard to provide framed data to a physical medium dependency device, thephysical medium dependency device and the networking card to convert thedata to a format that is usable by a data fabric.
 12. The apparatus ofclaim 11, wherein the networking card to include a router interface andcorresponding physical medium dependency device slots.
 13. The apparatusof claim 1 wherein the first and the second frame dimensions areprogrammable to change a number of rows and columns within therespective first and second frame dimensions without changing a numberof planes.
 14. A method, comprising: enabling access to frame dimensioninformation from a frame dimension unit, the frame dimension unitincluding a programmable plane counter, row counter, and column counter;and converting a data stream to and/or from a frame format with a SONETframer based on a frame dimension programmed into the frame dimensionunit, wherein the frame dimension includes at least one of aprogrammable number of rows per frame and a programmable number ofcolumns per frame to provide a variable frame size, wherein theprogrammable number of rows per frame and the programmable number ofcolumns per frame may be programmed without programming a change in aprogrammable number of planes per frame.
 15. The method of claim 14wherein the frame dimension comprises a programmable number of planesper frame.
 16. The method of claim 15 wherein the number of planescorresponds to one of a plurality of standard SONET data rates supportedby the SONET framer.
 17. The method of claim 14, wherein the SONETframer supports a range of data rates corresponding to at least one ofthe programmable number of rows and the programmable number of columns.18. An apparatus, comprising: means for enabling access to a size of anoptical data network frame via one or more of a plurality ofprogrammable counters in a frame dimension unit for a SONET frame,wherein the optical data network frame is to be transmitted and/orreceived over an optical network; and means for converting a data streamto and/or from a frame format based on a reprogrammed size of theoptical data network frame, wherein a variable frame dimension isprovided by a programmable plane counter, row counter and column counterin the frame dimension unit.
 19. The apparatus of claim 18 wherein themeans for enabling access to the size of the optical data network frameincludes one or more dip switches to represent one or more possiblemodes of operation.
 20. The apparatus of claim 18 wherein the means forenabling access to the size of the optical data network frame includesmeans configured to directly write a maximum number of planes to a planecounter.
 21. The apparatus of claim 18 wherein the means for convertingthe data stream to and/or from a frame format includes a SONET framerconfigured to fill available payload byte locations in a frame withbytes from the data stream and fill byte locations reserved for overheadin the frame with bytes generated by an overhead processor.
 22. Theapparatus of claim 18, further comprising an input/output (I/O) portcoupled to the means for converting the data stream to and/or from theframe format.